The invention relates to an implantable, multi-function medical device, particularly a combination cardiac pacemaker/defibrillator, in which different therapeutic functions place different load requirements on the battery. This can relate to the available current intensity or the available power.
In particular, a first function may require a low current in the .mu.A range over extended periods of time, and a second function may require a short-time pulse current in the Ampere range over several seconds for certain events occurring at irregular intervals.
Known energy sources for meeting such requirements are commercially available and are implemented in the field of human medicine. These energy sources are corresponding electrochemical systems which, depending on the structural design, are capable of emitting current intensities in ranges between 10.sup.-6 A and almost 10 A.
The electrochemical system of lithium/silver vanadium oxide is particularly suited for fulfilling these conditions; details about the system are given in U.S. Pat. Nos. 4,310,609, 4,391,729 and 5,114,811, as well as European Patent Application EP 0 638 946 A2.
Batteries using the lithium/vanadium system are also known: refer to U.S. Pat. Nos. 3,655,585 and 3,947,289 and R. J. Horning et al, New Rate Lithium/VanadiumPentoxideCell for Implantable Medical Devices, Progress In Batteries & Solar Cells, Vol. 4, 1982, pages 97-102.
A further practical system that comprises lithium, manganese oxide, lead oxide and chromium oxide is described in DE 4 438 784 A1, and is the subject of EP 0 777 286 B1.
Known batteries contain metal-oxide and lithium electrodes in an organic electrolyte. Such electrochemical systems have a maximum load of 30 mAcm.sup.2. Output powers in the order of magnitude of 10-W pulses can thus only be attained by battery constructions having a large surface area. Because of the large proportion of passive components in such batteries, the energy density in Wh/l and Wh/kg is relatively low.
A disadvantage shared by all of the aforementioned electrochemical systems is that they possess a relatively low energy density in Wh/kg and Wh/l because of the power requirements in different load ranges. Consequently, either the mass and volume of the battery are very high for an acceptable service life, or small and lightweight devices must be replaced relatively frequently.
U.S. Pat. No. 4,134,408 discloses a power-supply arrangement for an implantable pacemaker, which includes, in addition to the conventional (primary) battery, means for temporary external energy supply, and an associated (secondary) battery that has a low capacity and can be charged externally, and can then supply the pacemaker for a limited time in order to preserve the primary battery.
U.S. Pat. No. 5,591,212 discloses a so-called hybrid battery arrangement for an implantable pulse generator, especially an implantable defibrillator; in addition to the battery, the arrangement includes a capacitor that has a high energy density and is selectively connected to the battery for ensuring the supply of the control electronics of the device during operating phases requiring a large amount of current. A similar solution for a pacemaker had already been described in U.S. Pat. Nos. 4,416,282 and 4,590,941. These documents also teach the provision of an emergency backup battery, which can be activated in the event of a sudden voltage drop of the primary battery.
Providing a costly and voluminous high-capacity capacitor solely for the purpose of current management (without the capacitor being able to supply additional energy) is, in the end, as equally unsatisfactory as providing an emergency battery that, as such, permits neither an adaptation to different current requirements nor a significant extension of the service life.
With reference to older, two-battery arrangements for pacemakers, PCT Application WO 94/02202 describes an implantable cardioverter/defibrillator in which special batteries are provided for observation functions, on the one hand, and for emitting stimulation energy, on the other hand. These batteries are optimized to have a long service life with very low current intensities, on the one hand, and to provide a high peak current and an extremely low self-discharging, on the other hand. In particular, a pair of lithium/iodine batteries and lithium/vanadium pentaoxide batteries is used. This device has only one therapeutic function; furthermore, the inflexible allocation of the battery functions is unsatisfactory.